The Stirling thermodynamic cycle and heat engines employing same are well known in the art. Stirling engines typically utilize reciprocating pistons to convert heat energy introduced into the engine to mechanical or electrical energy. Stirling engines are basically of two types, the free piston type or the kinematic type. In a free piston Stirling engine, the engine housing or vessel is hermetically sealed to contain an engine working fluid and freely moving pistons which are not mechanically connected either to one another or to an external drive. A kinematic engine is one in which the engine housing is not hermetically sealed and in which there are mechanical connections both between engine pistons and to means external to the engine for extracting the engine's mechanical energy. Thus, a kinematic-type Stirling engine is best suited for automotive applications.
The first step of the Stirling thermodynamic cycle consists of an isothermal expansion of the working fluid contained in the engine's expansion space. In accordance with the ideal Stirling engine cycle, expansion of the working fluid should be effected isothermally. However, conventional Stirling engines, both of the free piston and kinematic type, are designed such that thermal energy is imparted to the working fluid before it enters the expansion space, and, largely as a consequence, idealized isothermic expansion of the working fluid is not realized.
For example, referring to FIG. 1, a free piston Stirling engine 20 typically comprises a displacer piston 22 and a compression piston 24 mounted on a common axis 26 within a hermetically sealed engine pressure vessel 28 for reciprocation in cylinders 30 and 32, respectively. A working fluid passes from a compression space 34, proximate one end of the compression piston, through a cooler 36, a regenerator 38 and a heater 40 into an expansion space 42 proximate one end of the displacer piston. A linear alternator 43 may be adapted to operate with compression piston 24 for extracting usable electrical energy. The heater, regenerator and cooler are each of a annular configuration disposed around the displacer cylinder. A typical prior art heater consists of a large number of small diameter tubes 44, arranged substantially parallel to axis 26 and through which the working fluid is conveyed between the compression and expansion spaces. For example, in a typical 25 KW Stirling engine as many as 1600 tubes, each of 1/16 inch inside diameter, are used. A heated liquid is pumped through the heater via inlet 40a and outlet 40b to contact the external tube surfaces in order to heat the working fluid flowing within the tubes. Thus, as can be seen in FIG. 1, thermal energy is introduced into the working fluid before it enters the expansion space where it undergoes the expansion step of the Stirling thermodynamic cycle. The engine working fluid is typically a gas such as hydrogen or helium. Thus, during the expansion of the gas, the gas temperature drops in accordance with the indirect relationship between gas temperature and volume. As a result, the expansion step is not as isothermal as called for in an ideal Stirling cycle, and engine operating efficiency suffers. It is thus desirable to isothermalize as much as possible the expansion step of the Stirling engine cycle.
The large number of tubes used in prior art heater heads also necessitates a net increase in the total volume of working fluid required in a Stirling engine. As is well understood in the art, that portion of the working fluid volume which is not swept by the pistons constitutes void volume and does not contribute to useful work. As void volume increases, the specific power output of a Sterling engine decreases. It is therefore desirable to minimize the working fluid void volume.
In the typical Stirling engine heater, such as diagrammatically illustrated in FIG. 1, a heated fluid is pumped through the void space between and about these tubes in order to introduce heat energy into the working fluid. Thus, the heated fluid, typically a molten metal such as lithium, normally contacts the wall of housing 28, as well as piston cylinder 30. Materials used in the constructions of housing 28 and cylinder 30 therefore must be of a character capable of withstanding the molten metal. Refractory materials such as niobium-1-Zirc alloy are most compatible with molten lithium. However, housing 28 and cylinder 30 may be required to withstand substantial pressures during engine operation, and refractory materials typically tend to be brittle and can lose their strength in contact with trace impurities typically present in the working fluid. Further, refractory materials are not castable and must therefore be machined to the desired shape and dimensions. It would be preferable to cast the housing and cylinder in order to simplify and cost-improve the manufacturing operation. Additionally, refractory materials are difficult to joint together.
The class of materials referred to as superalloys, e.g. MAR-M247, are castable and posses sufficient mechanical strength to withstand engine operating pressures and temperatures. Further, superalloys are compatible with the typical engine working fluids. However, superalloys are not compatible with molten metals such as molten lithium and for this reason are not appropriate as an engine housing material. Thus, prior art heater designs present engine material and fabrication shortcomings.